Materials capable of absorbing energy are needed for many safety, medical and orthopedic applications such a helmets, body protection pads and playground surfaces. In order that the material may absorb the maximum amount of energy and have a minimum rebound, which returns the energy to the head or body thereby causing injury, the foam should be of the lowest possible resilience. Energy absorbing materials are typically divided into four categories on the basis of their hardness: semi-rigid, semi-flexible, flexible and elastomeric. Semi-rigid foams are typically made from either expanded polystyrene or polyurethane. Semi-flexible and flexible foams are mainly made of polyurethane. The materials that are the topic of this invention are semi-flexible, flexible, semi-rigid and integral skin foams, and elastomeric polyurethanes with exceptionally low resilience.
Typical energy absorbing polyurethanes are produced from polyether polyols and an isocyanate and have high resiliency. The polyether polyols typically have an average molecular weight of 100 to 20,000 and an average functionality of from 2.4 to 2.7 hydroxyl groups per molecule. Toluene diisocyanates or diphenyl methane diisocyanate are used to produce foams and elastomers with a broad range of properties. The isocyanate functionality is typically from 2.0 to 2.3 isocyanate groups per molecule. For a given formulation the total diisocyanate groups are equivalent to, or in a slight excess, relative to the total number of hydroxyl groups.
In the past, castor oil was widely used in order to reduce the resiliency of polyurethane foams however this has been largely replaced by synthetic plasticizers. According to Syzycher's “Handbook of Polyurethanes”, Michael Syzycher, CRC Press, LLC, 1999, Chapter 8.5.5), foams prepared with castor oil have a tendency to shrinkage, and should be coated or otherwise protected from water due to their open cell structure. Thus there is disclosed in U.S. Pat. No. 4,987,156 a shock-absorbing polyurethane foam in which liquid plasticizers such as adipate, maleate and phosphate esters were added to improve the low temperature resiliance. The upper limit of use of the plasticizers was 150 pph. Above this the plasticizer inhibited the reaction between the polyols and the isocyanate. U.S. Pat. No. 5,128,381 proposes use of a mixture of plasticizers such as alkyl phenols and hydroxyalkyl phthalate esters. A disadvantage of the use of plasticizers is their tendency to sweat out of the foam causing a loss of properties and an unpleasant sticky sensation on contact with them. Another type of additive used to reduce resilience is asphalt as is exemplified in Japanese disclosures 152740/1986 and 15433/1984. However these rams had a narrow temperature range of application, and their energy absorbtion was limited to essentially room temperature. Other additives which have been utilized in the production of polyurethane foams include: phase change materials such as high hydrocarbons, disclosed in U.S. Pat. No. 5,677,048; chain extenders, disclosed in U.S. Pat. No. 5,047,494, water swellable fillers such as lignite and peat, disclosed in U.S. Pat. No. 4,734,439; silicone containing surfactants, disclosed in U.S. Pat. No. 4,554,295; organosilicone oils, disclosed in U.S. Pat. No. 3,926,866; and dispersions of organic and inorganic fillers less than 7 microns in size, disclosed in U.S. Pat. No. 4,243,755.
In U.S. Pat. No. 5,849,806 a low resilience polyurethane foam was produced using a special polydiene diol/mono-ol mixture, a tackifier and an oil in order to achieve good adhesion to substrates such as paper and tapes. These formulations, while producing low resilience foams suffer from a number of process limitations such as very high viscosities which necessitated preheating to 150° C. prior to and casting, and the use of special diols and mono-ols. Neither of these conditions are common practice in the polyurethane industry and therefore these formulations are of limited applicability. The inventors did not teach the applicative potential of these low resilience foams in applications other than adhesive tapes and sealants.
There is therefore a need for a low resilience polyurethane foam with a wide temperature range of application, which call be manufactured using conventional triols, and under processing conditions common in the polyurethane foam industry.
It is the object of the present invention to provide a series of low resilience foams with excellent energy absorbing properties that may be prepared from standard triols and/or diols, isocyanates and tackifying agents. The prepolymers are of low viscosity, easily mixed at room temperature and processed under regular industrial conditions.
These and other objects of the present invention will become more apparent from the summary of the invention and the detailed description of the drawings that follow.